The code for converting objects to QVariantMap was causing exception,
that was "leaking" to the next evaluation. One situation was reading
the property 'localStorage' when we do not have a proper security
origin, which throws a SECURITY_ERR.

Now, we will simply not include on the QVariantMap those properties,
and make sure that we clean the exception if necessary.

* bridge/qt/qt_runtime.cpp:
(JSC::Bindings::convertValueToQVariantMap):
Extracted function that performs conversion from JSObject to a QVariantMap. This
functions makes sure that exception is clean after its execution.

(JSC::Bindings::convertValueToQVariant):
Use the previous function. Add a comment explaining the choice of distance value.